Systemic administration of cocaine, given alone or in combination with sensory stimuli, differentially affects L-arginine-nitric oxide metabolism in discrete regions of the brain of rat

Further proof on the role of accumbal nNOS in cocaine-seeking behavior in rats

BACKGROUND

Cocaine use disorder (CUD) remains a severe health problem with no effective pharmacological therapy. One of the potential pharmacological strategies for CUD pharmacotherapy includes manipulations of the brain glutamatergic (Glu) system which is particularly involved in drug withdrawal and relapse. Previous research indicated a pivotal role of ionotropic N-methyl-D-aspartate (NMDA) receptors or metabotropic receptors' type 5 (mGlu5) receptors in controlling the reinstatement of cocaine. Stimulation of the above molecules results in the activation of the downstream signaling targets such as neuronal nitric oxide synthase (nNOS) and the release of nitric oxide.

METHODS

In this paper, we investigated the molecular changes in nNOS in the prefrontal cortex and nucleus accumbens following 3 and 10 days of cocaine abstinence as well as the effectiveness of nNOS blockade with the selective enzyme inhibitor N-ω-propyl-L-arginine hydrochloride (L-NPA) on cocaine seeking in male rats. The effect of L-NPA on locomotor activity in drug-naïve animals was investigated.

RESULTS

Ten-day (but not 3-day) cocaine abstinence from cocaine self-administration increased nNOS gene and protein expression in the nucleus accumbens, but not in the prefrontal cortex. L-NPA (0.5-5 mg/kg) administered peripherally did not change locomotor activity but attenuated the reinstatement induced with cocaine priming or the drug-associated conditioned cue.

CONCLUSIONS

Our findings support accumbal nNOS as an important molecular player for cocaine seeking while its inhibitors could be considered as anti-cocaine pharmacological tools in male rats.

Cocaine potentiates MDMA-induced oxidative stress but not dopaminergic neurotoxicity in mice: implications for the pathogenesis of free radical-induced neurodegenerative disorders

RATIONALE

The drugs of abuse 3,4-methylenedioxymethamphetamine (MDMA; "ecstasy") and cocaine both increase the generation of free radicals, and in the case of MDMA, this increase in oxidative stress is involved in the dopaminergic neurotoxicity produced by the drug in mice. Oxidative stress processes are also involved in the pathogenesis of several neurodegenerative diseases.

OBJECTIVES

We aimed to determine the consequences of the combined administration of MDMA and cocaine on oxidative stress and dopaminergic neurotoxicity.

METHODS

Mice received MDMA (20 mg/kg, i.p.; two doses separated by 3 h) followed by cocaine 1, 3, 6, or 24 h after the second MDMA dose. Mice were killed between 1 h and 7 days after cocaine injection.

RESULTS

MDMA decreased dopamine transporter density and dopamine concentration 7 days later. Cocaine did not alter this neurotoxicity. MDMA produced an increase in the concentration of 2,3-dihydroxybenzoic acid in striatal microdialysis samples and an increase in lipid peroxidation in the striatum which were potentiated by cocaine. MDMA and cocaine given together also increased nitrate and 3-nitrotyrosine levels compared with either drug given alone. On the other hand, MDMA increased superoxide dismutase activity and decreased catalase activity, changes which were prevented by cocaine administration. In addition, cocaine administration produced an increase in glutathione peroxidase (GPx) activity in both saline-treated and MDMA-treated mice.

CONCLUSIONS

Cocaine potentiates MDMA-induced oxidative stress but does not produce an increase in the neurotoxicity produced by MDMA, and this lack of potentiation may involve an increase in GPx activity.

Involvement of nNOS/NO/sGC/cGMP signaling pathway in cocaine sensitization and in the associated hippocampal alterations: does phosphodiesterase 5 inhibition help to drug vulnerability?

RATIONALE

Repeated cocaine administration induces behavioral sensitization in about 50 % of treated animals. Nitric oxide could be involved in the acquisition and maintenance of behavioral cocaine effects, probably by activation of neuronal nitric oxide synthase (nNOS)/NO/soluble guanylyl cyclase (sGC)/cyclic guanosine monophosphate (cGMP) signaling pathway, since inhibition of the nNOS enzyme attenuates development of sensitization in rats. On the other hand, increased cGMP availability by phosphodiesterase 5 inhibitors has been correlated to the misuse and recreational use of these agents and also to the concomitant use with illicit drugs in humans. Hippocampus is an important brain region for conditioning to general context previously associated to drug availability, influencing drug-seeking behavior and sensitization. Moreover, cocaine and other drugs of abuse can affect the strength of glutamate synapses in this structure, lastly modifying neuronal activity in main regions of the reward circuitry.

OBJECTIVE

The objective of this study is to determine whether the pharmacological manipulation of nNOS/NO/sGC/cGMP signaling pathway altered changes induced by repeated cocaine exposure.

RESULTS

The present investigation showed a relationship between behavioral cocaine sensitization, reduced threshold to generate long-term potentiation (LTP) in hippocampal dentate gyrus, and increased nNOS activity in this structure. However, when nNOS or sGC were inhibited, the number of sensitized animals was reduced, and the threshold to generate LTP was increased. The opposite occurred when cGMP availability was increased.

CONCLUSION

We demonstrate a key role of the nNOS activity and NO/sGC/cGMP signaling pathway in the development of cocaine sensitization and in the associated enhancement of hippocampal synaptic transmission.

Cocaine administration produces a protracted decoupling of neural and haemodynamic responses to intense sensory stimuli

Evidence suggests that for relatively weak sensory stimuli, cocaine elevates background haemodynamic parameters but still allows enhanced neural responses to be reflected in enhanced haemodynamic responses. The current study investigated the possibility that for more intense stimuli, the raised background may produce a protracted attenuation of the haemodynamic response. Three experiments were performed to measure effects of i.v. cocaine administration (0.5 mg/kg) or saline on responses in rat barrel cortex to electrical stimulation of the whisker pad. The first experiment used optical imaging spectroscopy (OIS) and laser Doppler flowmetry (LDF) to measure haemodynamic changes. Cocaine caused an increase in baseline blood flow (peak approximately 90%), which lasted for the duration of the test period (25 min). Haemodynamic responses to whisker stimulation were substantially reduced throughout. The second experiment used a 16-channel multi-electrode to measure evoked potentials at 100 mum intervals through the barrel cortex. Summed neural responses (collapsed across the spatial dimension) after cocaine administration were similar to those after saline. The third experiment extended experiment 1 by examining the effects of cocaine on whisker sensory responses using functional magnetic resonance imaging (and concurrent OIS or LDF). Cocaine caused a similar increase in baseline and reduction in the evoked response to that seen in experiment 1. Together, the results of these three experiments show that cocaine produces a protracted decoupling of neural activity and haemodynamic responses to intense sensory stimulation, which suggests that imaging techniques based on changes in haemodynamic parameters may be unsuitable for studying the effects of cocaine on sensory processing in humans.

Dose-related alteration in nitric oxide synthase mRNA expression induced by amphetamine and the full D1 dopamine receptor agonist SKF-82958 in mouse striatum

Neuronal nitric oxide synthase (nNOS) is normally expressed in one population of intrinsic interneurons of the striatum. Production of nitric oxide in the nNOS-containing neurons is sensitive to dopamine stimulation. Using quantitative in situ hybridization, the present study investigated the alteration in basal nNOS mRNA expression in striatal nNOS-containing neurons of mice treated with the psychostimulant amphetamine or a full D1 dopamine receptor agonist, SKF-82958. A single systemic injection of amphetamine induced a dose-related change in striatal nNOS mRNA expression. Whereas amphetamine at 4 mg/kg decreased basal levels of nNOS mRNA in both the dorsal (caudoputamen) and ventral (nucleus accumbens) striatum, the drug at a higher dose (12 mg/kg) increased nNOS expression in the two regions. Similarly, an acute systemic injection of SKF-82958 decreased and increased nNOS mRNA levels in the dorsal and ventral striatum at 2 and 4 mg/kg, respectively. These data indicate that constitutive nNOS expression in nitric oxide-producing neurons of the mouse striatum is regulated by dopaminergic transmission. Altered nNOS expression may result in changes in nitric oxide synthesis and thus contribute to biological actions of dopamine stimulants.

Involvement of nitric oxide in morphine-induced c-Fos expression in the rat striatum

Induction of expression of immediate-early gene c-Fos in the striatum is a common effect of many drugs of abuse, including morphine. Previous studies have shown that the morphine-mediated c-Fos response is attenuated by antagonists of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor. Other evidence suggests that the NDMA receptor may be coupled to the enzyme neuronal nitric oxide synthase (nNOS). NMDA receptor-mediated increases in intracellular calcium can activate nNOS, which catalyzes the formation of the signaling molecule nitric oxide. Because activation of NMDA receptors mediates morphine-induced c-Fos expression, we tested the hypothesis that activation of nNOS is involved in this cascade. Male rats were injected with the nNOS-selective inhibitor 7-nitroindazole (7-NI) or vehicle 30 min prior to injection of morphine sulfate or vehicle. Two hours later they were perfused with fixative and the brains removed for immunocytochemical analysis for c-Fos. Morphine induced c-Fos expression in the striatum, cerebral cortex, and midline/intralaminar nuclei of thalamus. Expression in the striatum, but not thalamus or cortex, was significantly blocked by 7-NI. Double-label immunocytochemistry revealed no co-localization of c-Fos and nNOS in any brain region. These results support a role for nNOS in the neural circuits activated by morphine.

Effect of melatonin on cocaine-induced behavioral sensitization

Melatonin, a pineal hormone and a potent free radical scavenger with neuroprotective actions, has been reported to act as an inhibitor of nitric oxide synthase (NOS). We have earlier shown that inhibitors of NOS (N(omega)-nitro-L-arginine methyl ester [L-NAME], 7-nitroindazole [7-NI]) block cocaine-induced behavioral sensitization. In the present study, the effects of melatonin on cocaine behavior were studied. A single injection of melatonin markedly augmented cocaine-induced locomotor activity. Rats injected daily with melatonin prior to cocaine injections failed to elicit cocaine sensitization. These behavioral effects of melatonin do not completely mimic those of other NOS inhibitors, suggesting that the effects of melatonin on cocaine behavior are mediated by both NOS-dependent as well as NOS-independent mechanisms.